Section 5 — Mutagenesis 



their radioresistant parents. This increase we 

 explain as the result of an additional effect of 

 heterosis. In the 4 x forms, dominant inheritance 

 of radioresistance cannot be doubted, but it is 

 somewhat more feebly expressed than in the 

 diploids. The radioresistance of the 4 hybrids 

 is somewhat lower than that of their radio- 

 resistant parents, but it is markedly higher than 

 the radioresistance of the initial forms (non- 

 selected). 



5.102. On the Possibility of the Modification of the 

 Ionizing Irradiation Genetical Effect in Barley. 



S. I. Yanushkevich (Moscow, U.S.S.R.). 



The possibility of the modification of the 

 A-rays (Co 60 ) effect on barley (Hordeum 

 sativum L. ssp. distichum) seeds in the dormancy 

 state under influence of condition preceding and 

 following the irradiation is discussed. In the 

 experiments the influence of the agro-climatic 

 conditions on the growing plants and of the 

 modification of nutrition conditions of the 

 barley seed embryos (by means of homo- and 

 heterogenous transplantation of embryo on the 

 endosperm) upon the radiosensibility, chromo- 

 some aberrations frequency and chlorophyll 

 mutations in M2 was under study. 



It was shown that plant growing in different 

 agro-climatic areas led to the radiosensibility 

 change of the seeds in the dormancy state. The 

 differences in radiosensibility could vary in large 

 limits and their manifestation was correlated to 

 the moisture level of irradiated seeds. The ad- 

 ditional data were obtained on the periodical 

 change of the radiosensibility of the seeds in the 

 state of dormancy kept under the conditions 

 of laboratory. 



The growing of Mi plants from irradiated 

 seeds in sharply different agro-climatic areas led 

 to definite variations of chlorophyll mutations 

 frequency in the progeny. 



The influence of nutrition conditions of 

 developing embryos of barley seeds on the 

 irradiation effect was noted. The nutrition of the 

 irradiated embryo by the non-irradiated en- 

 dosperm substances led to the change of the 

 chromosome aberrations total amount. The 

 irradiated endosperm, in its turn, induced chro- 

 mosome aberrations in the cells of growth zone 

 of non-irradiated embryos rootlets. 



The results of these experiments are discussed 

 in the light of the final effect dependence upon 

 the irradiated organism physiological state and 

 metabolism in the periods before and after 

 irradiation. 



5.103. Variable Influence of Humidity on Radio- 

 sensitivity of Seeds from Eight Botanical 

 Families. Thomas S. Osborne, Milton J. 

 Constantin, and Allyn O. Lunden (Oak 

 Ridge, U.S.A.). 



Classical studies with the barley grain {Hor- 

 deum) showed that superdry embryos are 

 extremely radiosensitive, becoming more re- 

 sistant by a factor of 4 or 5 as preirradiation 

 relative humidity (RH) approaches 50 per cent 

 and staying on a plateau of resistance even when 

 RH exceeds 90 per cent. We equilibrated various 

 dormant embryos at RH's of 10 to 85 per cent 

 then exposed them to gamma rays at doses of 

 to 400 kr. Dry weight of seedlings in controlled 

 environment rooms was measured. Families 

 studied were Compositae, Cruciferae, Cucur- 

 bitaceae, Gramineae, Leguminosae, Linaceae, 

 Solanaceae, and Umbelliferae. 



Embryos of the Gramineae (Hordeum and 

 Festuca) proved to be exceptional in that they 

 showed only trivial responses to variations in 

 preirradiation humidity. All other families gave 

 immense changes in sensitivity, even more than 

 200-fold, with changes in RH. There was a peak 

 RH for maximum resistance, with sensitivity 

 increasing at RH's both above and below; 

 the optimum RH differed with species. The 

 inadequacy of current free radical-water hypothe- 

 ses to explain increased sensitivity above the 

 optimum RH is obvious. 



Computer techniques with quadratic and 

 cubic formulae were used and discussed. Parts 

 of the data have been accepted for publication 

 by Radiation Botany. 



Operated by the University of Tennessee 

 College of Agriculture for the U.S. Atomic 

 Energy Commission under Contract No. AT-40- 

 l-GEN-242. 



5.104. Cysteamine and Sensitivity to X-rays in 

 Barley. A. Moes (Gembloux, Belgium). 



In 1951, Bacq, Herve et alS 1 ^ discovered that 

 cysteamine and cystamine are remarkable pro- 

 tectors against radiations in mice. 



The protective action of cysteamine has been 

 confirmed in our experiments conducted since 

 1954 on barley. (2 > Our tests are made with 

 Piroline spring bailey; the X-ray apparatus 

 works at 30 kV. The inhibition of the develop- 

 ment of the first leaf issuing from the irradiated 

 seeds is determined by a growth test made when 

 the control reaches approximatively 75 mm. (3) 



92 



